Since a compound such as a (meth)acrylic acid is extremely polymerizable, polymerization of the same often occurs in the production process, thereby unavoidably causing a halt of an apparatus due to the polymerization, as well known. To prevent this, a polymerization inhibitor such as hydroquinone or phenothiazine is used, or a molecular oxygen containing gas is applied.
In a producing process of a (meth)acrylic acid or the like, a packed column is used as a purifying device, but it is difficult to avoid polymerization of a polymerizable compound in the packed column. In a packed column, a flooding phenomenon in which a liquid to flow down does not flow down as a flow velocity of a gas is increased thereby causing the liquid to consequently overflow occurs more often on a support plate section, rather than in packing layers. Therefore, polymerization occurs more often on a support plate section, rather than in packing layers. Accordingly, openings in a support plate are usually arranged so as to be more than voids of a packing layer, that is, a ratio of a total area of openings to a column cross-sectional area is usually set more than a percentage of voids of a packing layer. Nevertheless a polymer formed causes the openings of the support plate to be plugged. In the case where a polymer is formed in the packed column and the openings of the support plate are plugged, the operation of the packed column is stopped and the polymer is removed manually or chemically.